Depends on what you have done,
if you did something to her like beat up her boyfriend or anything ur out of luck till she cools down
if you yelled or said something behind her back, talk to her over msn for a while then phone then face to face talk.
anything else talk to her, ask what you did, if she replys "u know what u did" or something, just wait for her to cool down and then go ask her again...
Also, if you know any of her close friends ask them what they know, close friends will always know whats going on.
overall time will fix everything.